        div{
            width: 500px;
            height: 500px;
            border: 2px solid red;
            /* 背景图片：
            url被称为统一资源定位符
             */
            background-image: url(./bk.jpg);

            background-repeat: no-repeat;
            background-repeat: repeat-x;
            background-repeat: repeat-y;
            background-repeat: repeat;
            /* repeat默认值 */
        }